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The present invention is directed to compressors for cryogenic air separation. In particular, the present invention is directed to a combined service integrally geared compressor for cryogenic air separation.
Cryogenic oxygen production facilities initially produced oxygen at near atmospheric pressure and used inline centrifugal compressors or reciprocating piston compressors to compress the gas to the required pressure. Low cost, high pressure oxygen production facilities have been developed as liquid pumped plants. In these facilities, a liquid oxygen stream is pumped to the required pressure and vaporized against a stream of high pressure air. The high pressure air is typically compressed using either a separate air booster compressor or where a booster compressor service is combined with that of the air separation unit feed air compressor with an atmospheric suction as part of a multi service compressor. This approach has historically been the low cost approach primarily because the of high cost of oxygen compression and the need for a safety barrier, when compared to the cost of air booster stages and a liquid oxygen pump. Combined service integrally geared compressors are quite common in the industry where main air compression services and dry air compression and/or nitrogen compression services have been combined on one gear box. Cost and power savings can be significant when comparing a low pressure gaseous oxygen plant over a liquid pump plant. In a low pressure gaseous oxygen plant, the gaseous oxygen comes off of a low pressure column in the plant as a gas and is compressed to less than 50 psig. In a liquid pump plant, the presence of freezable materials must be addressed where factors may include, at a minimum, the cost of additional design reviews to the significant expense of the addition of hardware to reduce or eliminate the impact of impurities (larger front end clean up system, guard adsorbers or boiling liquid oxygen in a separate vessel).
Process plant compressors are typically radial compressors having a large diameter bull gear with meshing pinions upon the ends of which compression impellers are mounted. The multiple impellers within their own respective housings provide several stages of compression as desired. The bull gear and its meshing pinons are contained within a common housing. Consequently such compressors are known as integral gear compressors. The pinions may have differing diameters to best match the speed requirements of the compression impellers they drive. The compressed air between any two stages may be ducted to an intercooler, wherein it is cooled, thereby providing a more efficient compression process.
Some concepts are known where two or more compression duties are combined on a single compressor. For example, U.S. Pat. No. 5,901,579 (Mahoney et al.) discloses a compressor where the main air compression duty is combined on one machine with two compression wheels that share the air coming off of the main air compressor and compresses those streams to feed an air separation plant.
European Patent Application No. EP 0 672 877 A1 describes a machine that combines one or more high pressure air booster stages with one or more cryogenic expander all coup led to a gear box which is in turn coupled to a motor generator.
Air Products and Chemicals, Inc., Research Disclosure 40380, entitled xe2x80x9cIntegrated Air Booster and Oxygen Compression for Partial Pumped LOX Cyrogenic Air Separation Process Cycle,xe2x80x9d published in November of 1997, describes a machine that combines elevated suction dry air booster stages with oxygen compression stages.
Air Products and Chemicals, Inc., Research Disclosure 41763, entitled xe2x80x9cOxygen Enrichment of Air: Process Developments and Economic Trends,xe2x80x9d published in January of 1999, teaches numerous methods to increase the oxygen concentration based on a cryogenic process to produce a rich oxygen stream. Among other things, a pumped liquid oxygen process is taught where an air compressor is coupled to a boost compressor which are separate units whose shafts are connected to allow a single driver for the process.
U.S. Pat. No. 5,402,631 (Wulf) and U.S. Pat. No. 5,485,719 (Wulf) teach a system for supplying compressed air to a process plant using a combustor-turbine unit directly coupled to a bull gear meshing with pinions on which are mounted gas compression and expansion stages. Some stages compress a stream of air supplied to the combustor-turbine unit for combustion and to the process plant. Other stages expand or compress other gas streams directed to the combustor-turbine unit or to external applications.
U.S. Pat. No. 5,924,307 (Nenov) teaches a compressor assembly for cryogenic gas separation wherein the assembly comprises a compressor, an expansion turbine, and an electric motor integrally connected via a gear drive. This patent teaches a combination of a cryogenic turbine with an electric motor/generator and a compressor stage (or stages) in one device, with a gear case, to provide optimal operation of both the cryogenic turbine and the compressor.
However, none of these patents teaches a combined service integrally geared compressor for cryogenic air separation where the compressor is integrated with the air separation unit processes to obtain an overall cost and power benefit.
The object of the invention is to lower plant costs by taking advantage of recent changes that have taken place in the compression industry and by taking advantage of the acceptance of integrally geared compressors in oxygen service. The concept is to integrate the compressor with air separation unit cycles to obtain an overall cost and power benefit. These benefits can be magnified if coproducts are taken from the air separation unit. Cost reduction comes with developments that have lowered the cost of oxygen compression through the use of integrally geared compression and the simplification in plant design that naturally results from the use of direct oxygen compression as opposed to liquid pumping. Further benefits are identified when using this concept in conjunction with air separation units that use static liquid oxygen head to pressurize a stream of oxygen prior to the compression stage.
It is principally desired to provide a combined main air/O2 enriched product compressor that overcomes the limitations of the prior art.
It is further desired to provide a combined main air/O2 enriched product compressor that is highly efficient.
It is still further desired to provide a combined main air/O2 enriched product compressor that allows for a simple design.
It is further desired to provide a combined main air/O2 enriched product compressor where there is no requirement for a separate pump and all of its controls, piping and instrumentation.
It is still further desired to provide a combined main air/O2 enriched product compressor where there is no requirement for air booster stages.
It is also desired to provide a combined main air/O2 enriched product compressor where there is allowance for a possible reduction in heat exchanger cost.
It is further desired to provide a combined main air/O2 enriched product compressor that provides improved oxygen recovery.
It is still further desired to provide a combined main air/O2 enriched product compressor that provides decreased specific power where less energy is required to recover a unit amount of O2 enriched gas.
Finally, it is desired to provide a combined main air/O2 enriched product compressor which provides lower plant costs and power consumption by reducing the scope of, or by eliminating entirely, equipment associated with the removal of trace contaminants, (guard adsorbers, larger TSA systems, external vaporization pots), which promote the build up of hydrocarbons in the air separation unit.
A combined main air/O2 enriched product compressor for use with an air separation unit that produces O2 enriched product where the concentration of O2 is greater than air is provided that includes a prime mover that drives a bull gear. The bull gear drives at least two pinion gears, and the pinion gears drive several compression stages where at least one compression stage compresses feed air for the air separation unit and at least one compressor stage compresses O2 enriched product gas from the air separation unit. The combined main air/O2 enriched product compressor satisfies all air separation unit feed air requirements and at least some compression for the O2 enriched product gas from the air separation unit.
At least one compressor stage that compresses the O2 enriched product gas preferably compresses the O2 enriched product gas to no more than about 50 psig.
The compressor includes a feed section to draw in atmospheric air to be compressed in the compressor.
The compressor preferably compresses the atmospheric air to between 60 and 200 psia.
The pressure of the O2 enriched product gas provided by the air separation unit is preferably xc2xd to ⅙ the feed air pressure to the air separation unit.
A method for operating a cryogenic air separation unit is also provided, which includes the steps of providing a combined main air/O2 enriched product compressor for use with the air separation unit that produces O2 enriched product, where the product compressor includes a prime mover, a bull gear and at least two pinion gears. The steps further include driving the bull gear using the prime mover, driving at least two pinion gears with the bull gear, and driving a plurality of compressor stages with the pinion gears where at least one compression stage compresses feed air for the air separation unit and at least one compressor stage compresses O2 enriched product gas from the air separation unit. Again, the combined main air/O2 enriched product compressor satisfies all air separation unit feed air requirements and at least some compression for the O2 enriched product gas from the air separation unit.
Preferably, in the method, the step including compressing O2 enriched product gas from the air separation unit includes compressing the O2 enriched product gas to no more than about 50 psig.
Preferably, the step of providing the compressor includes providing a compressor feed section to draw in atmospheric air to be compressed in the compressor.
The step of compressing the atmospheric air preferably includes compressing the atmospheric air to between 60 and 200 psia.
The step including compressing O2 enriched product gas from the air separation unit preferably includes compressing the O2 enriched product gas to 1.2 to 7 times greater than the feed air pressure to the air separation unit.